I would go from 36 to 40, then 44, then 48, etc. I do 60 hour fasts now but it took months (maybe a year?) to build up to and I still have never attempted a 72 hour fast. Maybe by this summer I will be ready. Also I know Reddit preaches this but electrolytes really do help. Good luck!
Build up your nutrition before you attempt an extended fast, your body needs every advantage you can give it! To lay things out get in a bunch of high quality fat and protein say two? Days prior to your extended fast. I am no expert, and am not highly educated on this topic.